Echo Path Change Detection in a Network Echo Canceller

ABSTRACT

An echo canceller is proposed that has an echo path change detector ( 10 ) capable of reliably distinguishing an echo path change from the condition of double talk. The echo path change detector is adapted to sample filter parameters from an FIR adaptive filter in the echo canceller, and to detect a change in intensity in a pattern formed by the sampled filter parameters. An echo path change is signalled when the degree of said pattern intensity change exceeds a predetermined level. The invention resides in the understanding that the parameters or coefficients of the adaptive FIR filter react differently to a condition of double talk or echo path change. Specifically, when these parameters are sampled and represented as an intensity pattern, a significant change in this intensity pattern over time can indicate an echo path change.

FIELD OF INVENTION

The present invention relates to the cancelling of network echo ineither fixed or digital mobile telephone systems.

BACKGROUND ART

In today's digital mobile telephone systems, the two main factorsaffecting speech quality are network delay, resulting from thecombination of speech coding and channel coding, and network echo.Network echo is the result of a portion of the speech signal energy froma “far end” subscriber being reflected back to the sender. This iscaused primarily by the impedance mismatch in the 4-wire to 2-wireconversion hybrid in the public switched telephone network (PSTN)subscriber interface. Any network delay makes network echo moreapparent, and hence more irritating to the subscriber.

It is known to use an echo canceller to reduce or remove the networkecho and hence improve the overall speech quality. Conventional echocancellers use an adaptive finite impulse response (FIR) filter to modelthe echo path, generate a replica of the echo and subtract this from the“near end” signal to cancel the echo component. The adaptive FIR filtercoefficients or parameters are updated by a least mean square (LMS)algorithm to minimise the prediction error of the adaptive filter. Theadaptive FIR filter only models the linear portion of the echo.Non-linear effects from the conversion hybrid and the subscriber, suchas saturation and unsymmetrical distortion, limit the echo cancellerperformance to around 30 dB echo return loss. In order to remove thisresidual echo the echo canceller also uses a nonlinear processor.

When the echo path impulse response changes (hereinafter called “echopath change”), the FIR filter uses the prediction error to update theFIR filter coefficients or parameters. However, the FIR filter is unableto distinguish an echo path change from a condition called double talk,which arises when both subscribers talk at once. In this lattersituation, the residual signal is dominated by the “near end” signalrather than by echo. However, this effect, in a similar fashion to anecho path change, causes the prediction error to increase. This isproblematic because when double talk occurs, the FIR filter parametersshould be frozen to prevent deviation, rather than updated as they wouldbe for an echo path change. On the other hand, when an echo path changeoccurs, the adaptation rate should be increased in order to speed upconvergence of the adaptive FIR filter to remove the echo.

A number of methods for detecting the conditions of double talk and echopath change in an echo canceller have been proposed. C. Carlemalm, F.Gustafsson and B. Wahlberg, (1996) “On the problem of detection anddiscrimination of double talk and change in the echo path” Proc. ICASSP'96, Atlanta, pp 2742-2745 and C. Carlemalm, (1998) “On model-baseddetection and estimation schemes in statistical signal processing” Ph.D. Thesis, Royal Institute of Technology, Stockholm, both suggest usinga likelihood-based approach. Both the latter reference and C. Carlemalmand A. Logothetis (1997) “On detection of double talk and changes in theecho path using a Markov modulated channel model” Proc. ICASSP '97,Munich. present a hidden Markov model method. The use of such methods tomodel the echo path and then detect echo path change or double talkprovides good results in computer simulations, but still does not permit100% detection. Particularly in the last-mentioned reference, thedetection of echo path change appears easier than that of double talk.However, this comes at the expense of high computational complexity whenthis method is put into practice. The complexity of the likelihood-basedapproach has been estimated as roughly equivalent to that of tworecursive least square RLS filters; the complexity for the hidden Markovmodel method is reportedly less than this, but nevertheless is stilldifficult to implement in practice in a cellular network application.

WO 98/28857 describes a method for detecting echo path change and doubletalk by measuring and comparing the linear dependencies between theresidual signal and the echo estimate and between the residual signaland the desired signal. While this method is very effective it is unableto provide sufficiently rapid convergence when the path changes duringan established call, for example when the far end signal is switchedfrom one phone to another in some situations.

It is thus an object of the present invention to propose a method andapparatus for reliably distinguishing an echo path change from thecondition of double talk.

It is a further object of the present invention to propose a method andapparatus for providing an echo path change detector that can beimplemented with low computational complexity.

It is a further object of the present invention to propose a method andapparatus for detecting an echo path change with a rapid response.

It is a still further object of the present invention to present amethod and apparatus for echo cancellation that reliably and rapidlydistinguishes an echo path change from the condition of double talk.

SUMMARY OF THE INVENTION

The above objects are achieved in an arrangement and method as set outin the accompanying claims.

In accordance with one aspect, the present invention resides in an echocanceller for use in telecommunication system that has a finite impulseresponse adaptive filter with modifiable filter parameters and an echopath change detector coupled to the final impulse response filter. Theecho path change detector is adapted to sample the FIR filter parametersover time and to detect a change in a pattern formed by the sampledfilter parameters. An echo path change is signalled when the degree ofsaid pattern change exceeds a predetermined level. Preferably thedetected pattern change is a change in an intensity pattern formed bythe sampled filter parameters.

In essence, the present invention resides in the understanding that theparameters or coefficients of the adaptive finite impulse responsefilter of the echo canceller react differently when confronted with thecondition of double talk or with an echo path change, respectively.Specifically, when these parameters are sampled and represented as anintensity pattern, a significant change in this intensity pattern overtime can indicate an echo path change.

Preferably, the intensity pattern of the filter parameters isrepresented by the intensity levels in the areas of high intensity bydetermining the difference between the highest and lowest filterparameters for each set of sampled parameters. This greatly simplifiesthe generation of a signal representing the intensity pattern and alsoreduces the necessary computational capacity.

A gradual change in such areas of high intensity in the parameterintensity pattern is a characteristic of an echo path change and thuscontributes towards differentiating an echo path change from doubletalk. In accordance with a preferred embodiment of the invention, achange in this intensity pattern is determined by generating an averagelevel of these difference signals. When the newly calculated intensitysignal differs by a predetermined level from the long-term averagelevel, this indicates an echo path change.

Preferably, this long-term average is generated using a low-pass filter.The intensity signal should anyway be filtered to remove noise and otherartefacts, so the echo path change detector preferably includes two lowpass filters. A first low-pass filter has a short time constant suitablefor rejecting artefacts. A second low-pass filter has a longer timeconstant suitable for additionally generating a long-term averageintensity level. These two filter outputs are then compared and when thedifference exceeds a predetermined level, an echo path change issignalled.

The predetermined level may be a fixed level, but is preferably set as apercentage of the long-term average intensity signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will becomeapparent from the following description of the preferred embodimentsthat are given by way of example with reference to the accompanyingdrawings. In the figures:

FIG. 1 schematically illustrates an echo canceller including an echopath change detector in accordance with the present invention and

FIG. 2 schematically illustrates the equivalent circuit of aconventional echo canceller,

FIG. 3 depicts samples of parameters of the FIR filter in a conventionalecho canceller under the application of a DT test signal, whichsimulates the presence of low-level near end speech signals,

FIG. 4 depicts the same samples of parameters of the FIR filter as shownin FIG. 3 in the absence of low-level near end speech signals,

FIG. 5 depicts samples of the FIR filter parameters before and after anecho path change, and

FIG. 6 schematically depicts the functional elements of an echo pathchange detector in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified schematic representation of an echo canceller1 in accordance with the present invention. The echo canceller 1receives a first input signal, Rin, from a far end, FE, and a secondinput signal Sin at a near end, NE. The near end, NE, or cancelled end,of the echo canceller contains the echo path on which the echo cancellerI operates. This includes all transmission facilities and equipment thatare included in the echo path, including the 2-wire/4-wire hybrid 2 andterminating telephone set (not illustrated).

The main component of the echo canceller 1 depicted in FIG. 1 is anadaptive filter 20, which is a finite impulse response (FIR) filter.This filter 20 uses an adaptation algorithm to model the impulseresponse of the echo path and generate a predicted echo signal. Thispredicted signal is then subtracted at element 40 from the near endsignal Sin to cancel the echo component and generate an echo-free outputsignal Sout. A non-linear processor (NLP) 30 is used to remove residualecho that may remain after linear processing of the input signal. Itwill be understood by those skilled in the art that the echo cancellermay also contain other elements not illustrated here, such as a fixedfilter, DC filters and comfort noise generators.

Also depicted in FIG. 1 is an echo path change (EPC) detector 10. Thisis coupled to both the FIR filter 20 and the non-linear processor 30. Aswill be described below, the echo path change detector 10 detects anecho path change by analysing the parameters of the FIR filter 20.

The adaptive FIR filter parameters are updated using a least mean square(LMS) algorithm to minimise the prediction error of the FIR filter 20.The quality of the echo path estimation depends on the step size of thisadaptation algorithm. A small estimation error is generated by using asmall step size, but this comes at the cost of slow adaptation or“convergence” rate. A larger step size results in a higher adaptationrate. When a change in the echo path is signalled by the echo pathchange detector 10, this causes the modification of the step size usedin the LMS algorithm of the FIR filter 20, as well as a change in theclip level in the non-linear processor 30.

In accordance with the present invention, the detection of echo pathchange as distinguished from the condition of double talk is based onthe realisation that the FIR filter parameters are affected differentlyby these two conditions. This can be explained with reference to FIG. 2,which shows the equivalent circuit of a conventional echo cancellerwithout an echo path change detector.

In the diagram of FIG. 2, x(t) denotes the received (far end) signal;the near end signal is denoted by n(t), s(t) is the predicted echo onthe received signal x(t), y(t) is the received echo signal plus the nearend signal n(t), W is the weighting factor for each FIR parameters k attime t; H is the impulse response of the hybrid, and e(t) is theecho-cancelled received signal and is a combination of a residual(estimation error) signal and the near end signal.

The predicted echo on the received signal x(t) can be expressed asfollows:

$\begin{matrix}{{s(t)} = {\sum\limits_{k = 0}^{K - 1}{{W_{k}(t)}{x\left( {t - k} \right)}}}} & (1)\end{matrix}$

The desired signal y(t) may be expressed as follows:

$\begin{matrix}{{y(t)} = {{\sum\limits_{k = 0}^{K - 1}{{H_{k}(t)}{x\left( {t - k} \right)}}} + {n(t)}}} & (2)\end{matrix}$

Finally, the echo-cancelled received signal, e(t) can be expressed asshown in equation (3).

$\begin{matrix}{{e(t)} = {{{y(t)} - {s(t)}} = {{\sum\limits_{k = 0}^{K - 1}{\left( {{H_{k}(t)} - {W_{k}(t)}} \right){x\left( {t - k} \right)}}} + {n(t)}}}} & (3)\end{matrix}$

It is apparent from equation (3) that as W approaches H, the estimationerror, e(t), approaches the near end signal n(t) and will equal this inthe ideal condition. Echo cancellation is transparent to the near endand serves to isolate the far end signal. When either an echo pathchange or double talk occurs, the residual signal e(t) will be increasedin either the first or second part of equation (3). But, is it difficultto identify which of these conditions has occurred simply by evaluatingthe increased e(t). However, there is a difference in the way the FIRparameters are modified in these two conditions. Specifically, when anecho path change occurs, the impulse response of the hybrid, H, jumpsaway from the FIR parameters, W, after which the FIR parameters, W,track the changes in H. The same behaviour is not displayed in thecondition of double talk because the near end speech signal does notcontribute consistently to the FIR parameter estimation. Thisobservation opens up the possibility of detecting echo path changereliably and without confusing it with the condition of double talk bymonitoring and identifying the parameter behaviour.

This difference in behaviour of the filter parameters in response todouble talk or an echo path change is illustrated in FIGS. 3 to 5.

FIG. 3 shows four samples of 512 FIR filter parameters using testsignals defined in ITU-T recommendation G.168 Test 3A, which representdouble talk with low near-end speech levels. FIG. 4 shows the samesample times as in FIG. 3 in the absence of near end low-level speech.

FIG. 5 shows the effect on FIR parameters of a test signal representingan echo path change. This test is carried out in the absence of an echopath change detector. In FIG. 5, the EPC occurs at sample number 124400.The parameters are illustrated before the EPC (at sample 10000), 200 msafter the EPC (at sample 126000) and later still (at sample 200000).

It is apparent from a comparison of FIGS. 3 and 4, which respectivelyshow parameters in the presence and absence of double talk, that a nearend signal (low level speech) affects all parameters to a degree. Theeffect of an echo path change shown in FIG. 5, however, is markedlydifferent. The FIR parameters reproduce the impulse response of thehybrid. An echo path change essentially shifts or scales this response.The transition between the old impulse response and the new impulseresponse can be described as a change in intensity and/or position ofthe pattern formed by the FIR parameters. A high intensity object fadesout at one position while a new high intensity object fades in at thesame or a new position.

By viewing the FIR parameters as a one-dimensional intensity pattern itis possible to identify the occurrence of an echo path change as aconsistent change in intensity, position or both, of the parameterpattern.

The echo path change detector according to the present invention uses achange in the pattern intensity of the FIR parameters to detect an echopath change reliably. The echo path change detector 10 is illustratedschematically in FIG. 6. As shown in the figure, this detector includesa pattern intensity generator 101, two low pass filters 102, 103,connected in parallel to the output of the pattern intensity generator,a difference calculator 104 and a comparator 105 arranged to compare theoutputs of the low pass filters 102, 103 and generate an output signalindicative of a detected echo path change. This output signal is thensent to the FIR filter 20 and the non-linear processor 30 illustrated inFIG. 1. The function and implementation of these elements or functionalmodules is described in more detail below.

The pattern intensity generator 101 receives the FIR parameter valuesfrom the FIR filter 20 shown in FIG. 1 and generates a signalrepresentative of the intensity pattern of these parameters.

The properties of a two-dimensional pattern in terms of its intensity,position, area and the like may be described using the method ofmoments. The moment of an object is defined as

$\begin{matrix}{M_{pq} = {\sum{\sum\limits_{{({x,y})} \in R}{x^{p}y^{q}{f\left( {x,y} \right)}}}}} & (4)\end{matrix}$

where p+q represents the order of the moment, x and y are pixelcoordinates, f(x,y) represents the pixel intensity function and R is thespace where the object is located.

To reduce computational complexity, it is possible to use only the zeroand first order moments to describe the FIR parameters. These are:

$\begin{matrix}{M_{00} = {\sum{\sum\limits_{{({x,y})} \in R}{f\left( {x,y} \right)}}}} & (5) \\{{M_{10} = {\sum{\sum\limits_{{({x,y})} \in R}{{xf}\left( {x,y} \right)}}}}{and}} & (6) \\{M_{01} = {\sum{\sum\limits_{{({x,y})} \in R}{{yf}\left( {x,y} \right)}}}} & (7)\end{matrix}$

The centroid coordinates of the object, i.e. the center of gravity ofthe intensity pattern, are then:

$\begin{matrix}{x_{c} = {{\frac{M_{10}}{M_{00}}\mspace{14mu} {and}\mspace{14mu} y_{c}} = \frac{M_{01}}{M_{00}}}} & (8)\end{matrix}$

In order to reduce the computational complexity required in an echo pathchange detection algorithm the pattern intensity generator 101 performsa modified version of equations (4) to (8) to obtain the instantaneouspattern intensity, I_(instant). This is represented by the followingequation:

I _(instant)=FIR_(max)−FIR_(min)  (9)

where FIR_(max) and FIR_(min) are the maximum and minimum FIR filterparameters (coefficients), respectively. In other words, the twobrightest pixels are used to represent the intensity of the FIRparameter pattern. The instantaneous pattern intensity indicates thelevel of the predicted echo on the received far end signal. Thecoordinates of the intensity pattern provide information on the delay ofthe echo path. This simplification of equations (4) to (8) permits theinstantaneous pattern intensity to be calculated in real time withoutgreat computational capacity.

Low-pass filter blocks 102 and 103, process the instantaneous patternintensity to obtain information extraction and artefact rejection, suchas the rejection of low-level near-end speech and noise. This isessentially a smoothing of the estimated FIR parameter intensitypattern, and has a function represented by the following equation:

$\begin{matrix}{{I(t)} = {{I\left( {t - 1} \right)} + \frac{\left( {I_{instant}{\operatorname{<<}16}} \right) - {I\left( {t - 1} \right)}}{2^{shift}}}} & (10)\end{matrix}$

This smoothing function defined in equation 10 is performed in each ofthe low pass filter blocks 102, 103 using 1^(st) order infinite impulseresponse (IIR) filters. In this equation 2^(shift) is the smoothingfactor, which can be achieved using shifts and without the need formultiplication. Incidentally, it is noted that the operator “<<” used inequation (10) enables the increase in calculation resolution whenperforming the division. Each low pass filter block 102, 103 applies adifferent smoothing factor. Filter block 102 has a longer time constantthan filter block 103. The respective time constants are selected suchthat filter block 102 generates a long-term average of the FIR parameterpattern, while filter block 103 generates the estimated instantaneousFIR parameter pattern intensity, filtered to remove noise and low levelnear end speech.

The outputs of blocks 103 and 102, representing the instantaneous andlong-term pattern intensity signals, respectively, are then compared inthe difference calculator 104 and comparator 105.

Measurements have shown that, in general, double talk generates a lessthan 10% change in the FIR parameter intensity pattern. Moreover, theeffect of double talk on the FIR parameter pattern is somewhat random,depending on the speech pattern. An echo path change, on the other hand,may generate a more than 30% change in the FIR parameter pattern for aspecific time interval, typically around 100 ms. Any low-level near endspeech and noise present on the FIR parameter pattern intensity signalscan be smoothed away by the filter functions. Any remaining signalscaused by the presence of a near-end speech signal are then ignored bysetting a threshold level. A change in the instantaneous FIR parameterpattern intensity level relative to the long-term FIR parameter patternintensity level that exceeds this threshold indicates that the echo pathhas changed. Accordingly, the difference between the long-term averageFIR parameter intensity and instantaneous FIR parameter intensity valuesoutput by the difference calculator 104 is compared in the comparator105 with a detection threshold level V_(th). If this threshold level isexceeded, an echo path change is flagged, permitting the modification ofthe step size used in the LMS algorithm of the FIR filter 20, as well asa change in the clip level in the non-linear processor 30.

This threshold level V_(th) may be an absolute, or fixed level.Alternatively and preferably, however, it is a percentage change of thelong-term average FIR parameter intensity value output by low passfilter 102. The threshold level is preferably set in the range of10-30%, and most preferably in the range of 20-25%, of the long-termaverage FIR parameter intensity level.

The computational capacity required for the echo path change detectionalgorithm may be reduced still further by performing echo path changedetection over 32 sample intervals, such that the detection algorithm isrepeated only for every 32 sample.

The echo path change detector 10 described above is preferablyimplemented in software in a digital signal processor (DSP). Since theecho canceller 1 itself is conventionally implemented as a softwarealgorithm using a DSP, the echo path change detector 10 can then form anextension of the echo canceller algorithm. Alternatively, those skilledin the art will readily recognise that the algorithm used by the echopath change detector 10 may be implemented in hardware, for example,using field programmable gate array (FPGA).

The echo canceller 1 including the path change detector 10 can beincorporated in the public switched telephone network PSTN. It can alsobe installed in an edge node of a core network portion of a digitalmobile communication network to remove the echo generated in the PSTN.In 3G networks, for example, the echo canceller 1 with the path changedetector 10 would be incorporated in a media gateway that connects themobile network to the PSTN.

1. An echo canceller for use in a telecommunication system, the echocanceller comprising: a finite impulse response adaptive filter having aset of modifiable filter parameters; and an echo path change detectorcoupled to said finite impulse response filter, wherein said echo pathchange detector is adapted to receive samples of said set of filterparameters over time, to detect a change in a pattern formed by saidfilter parameters sampled over time and to signal an echo path changewhen the degree of said pattern change exceeds a predetermined level. 2.The echo canceller according to claim 1, wherein the echo path changedetector includes a pattern intensity generator adapted to generate anintensity signal representing a pattern intensity of said sampled set offilter parameters, and an intensity change detector adapted to identifya change in said intensity signal over time.
 3. An echo canceller foruse in a telecommunication system, the echo canceller comprising: afinite impulse response adaptive filter having a set of modifiablefilter parameters; and an echo path change detector coupled to saidfinite impulse response filter, wherein said echo path change detectorincludes a pattern intensity generator adapted to receive samples ofsaid set of filter parameters over time and to generate an intensitysignal representing the pattern intensity of said sampled set of filterparameters, and an intensity change detector adapted to identify achange in said intensity signal over time and to signal an echo pathchange when the degree of said pattern intensity change exceeds apredetermined level.
 4. The echo canceller according to claim 3, whereinsaid pattern intensity generator is adapted to calculate the differencebetween a maximum and a minimum adaptive filter parameter values foreach set of sampled parameters.
 5. The echo canceller according to claim3, wherein said intensity change detector includes a first low passfilter having a first time constant, a second low pass filter having asecond time constant longer than said first time constant and acomparator connected to the output of said first and second low passfilters, wherein said first and second low pass filters are arranged toreceive said intensity signal and to output, respectively a short-termaverage and long-term average of said intensity signal, and saidcomparator is arranged to determine a difference between the short-termand long-term averages and to ascertain an occurrence of echo pathchange when the short-term average differs from the long-term average bya predetermined level.
 6. The echo canceller according to claim 5,wherein said predetermined level is set at a percentage of the output ofsaid second low pass filter.
 7. The echo canceller according to claim 5,wherein said predetermined level is set in a range of 10-30% of theoutput of said second low pass filter and preferably between 20-25% ofthe output of said second low pass filter.
 8. The echo cancelleraccording to claim 3, wherein said finite impulse response adaptivefilter includes an adaptation algorithm, and in that said adaptivefilter is adapted to respond to a signaled echo path change by modifyinga step size in said adaptation algorithm.
 9. The echo cancelleraccording to claim 3, further comprising at least one non-linearprocessor adapted to respond to a signaled echo path change by modifyinga clip level.
 10. A method for detecting an echo path change for use inan echo canceller having an adaptive finite impulse response filter,said method comprising the steps of: sampling sets of filter parameterfrom said adaptive finite impulse response filter over time, determininga change in intensity in an intensity pattern formed by said filterparameters sampled over time and signaling an echo path change when thedegree of said pattern intensity change exceeds a predetermined level.11. The method according to claim 10 wherein the step of determining achange in intensity in said filter parameter intensity pattern includesthe step of: generating an intensity signal representative of thepattern intensity of said sampled filter parameters.
 12. The methodaccording to claim 11, wherein said step of generating an intensitysignal includes the step of calculating the difference between a maximumand a minimum adaptive filter parameter values in each set of sampledfilter parameters to generate said intensity signal.
 13. The methodaccording to claim 11, wherein the step of determining a change inintensity in said filter parameter intensity pattern includes the stepsof: generating a long-term average of said intensity signal; anddetermining if said intensity signal differs from said long-term averageby said predetermined level.
 14. The method according to claim 13further comprising the step of low-pass filtering said intensity signalusing a first time constant to generate said long-term average and asecond time constant shorter than said first time constant to generatean instantaneous pattern intensity signal and comparing the outputs ofsaid first and second low pass filters to determine if saidinstantaneous pattern intensity signal differs from said long-termaverage by said predetermined level.
 15. The method according to claim13, further comprising the step of setting said predetermined level at apercentage of said long-term average of said intensity signal.
 16. Themethod according to claim 13, further comprising the step of settingsaid predetermined level at between 10-30% of said long-term average ofsaid intensity signal, and between 20-25% of said long term average ofsaid intensity signal.